Vitamin C is synthesized in plants and most higher animals from D-glucose. However, humans have a genetic defect that prevents the synthesizing of vitamin C. In this regard, humans must consume the required vitamin C in their diets or by supplementation. Because of the recognized need and demand for vitamin C, several different methods for synthesizing it are currently utilized, including the Reichstein process. The starting material for the Reichstein process is glucose, which is converted in several steps to 2-Keto-L-gulonic acid (KLG). KLG is esterified with methanol and acid to form the KLG methyl ester, which is then treated with sodium methoxide in methanol to give sodium ascorbate. Sodium ascorbate is then acidified with sulfuric or hydrochloric acid to give the ascorbic acid product plus an equimolar quantity of an unwanted waste salt.
To eliminate the need of introducing an acid into the ascorbate salt solution which forms a waste salt stream, U.S. Pat. No. 5,702,579 discloses a method of protonating ascorbate salts via electrodialysis using bipolar membranes and monopolar membranes. In this patent publication, the feed stream introduced into the electrochemical cell is limited to an ascorbate salt solution. By limiting the feed stream to just an ascorbate salt solution the conductivity of the electrochemical cell constantly decreases. As such, cell voltage during constant current operation will constantly increase to compensate for the decreased conductivity thereby increasing the overall cost of converting the ascorbate salt to ascorbic acid. Also, continuous conversion of ascorbate salts to ascorbic acid is not efficiently achieved because the presence of increased amounts of ascorbate salts in solution increases the solubility of ascorbic acid thereby rendering it less likely to precipitate out of solution and form fine powder crystals on a continuous basis.
Accordingly, simple electrochemical methods are needed which convert ascorbate salt to ascorbic acid without producing a stream of waste salt, which offer improved conductivity of the feed solution thereby reducing high voltage requirements, and which offer the ability to operate at quantitative conversion of ascorbate salt to ascorbic acid in both batch and continuous mode.